Snap-action pilot valve



F. w. scHNEcK 2,507,384

SNAP-ACTI ON PILOT VALVE May 9, 195o 2 Sheets-Sheet AIl Filed Jan. 1l,1946 INVENTOR.

F. W. SCHNECK ATTORNEY amil May 9, 1950 Filed Jan. ll, 1946 F. w.scHNEcK 2,507,384

SNAP-ACTION PILOT VALVE I 2 sheets-sheet 2 TIME INVENTOR. F. W. SCHNECKPatented May 9, 1950 SNAP-ACTION PILOT VALVE Frederick W. Schneck,Pacific Palisades, Calif., assigner to Bendix Aviation Corporation,South Bend, Ind., a corporation of Delaware Application January 11,1946, Serial No.` 640,466

6 Claims.

This invention relates to hydraulic valves of the pilot type and hasparticular application to pressure regulating valves for use inhydraulic power systems. The invention represents an improvement inpilot valves of the general type disclosed in the patent to W. C.Trautman, 2,320,763, issued June 1, 1943.

, An object of the invention is to provide a pressure responsive pilotvalve having a construction such that it opens and closes with a snapaction. Another object is to provide a pilot valve in which snap actionon opening and closing is obtained without the use of special mechanicalmechanisms.

' f Another object is to provide a pilot valve utilizing uid pressureforces for producing a snap action on opening and closing.

Another object is to provide a pilot valve having snap action on openingand closing that is simple and reliable and retains its desired snapaction operational characteristics irrespective of wear resulting fromlong service.

Another object is to provide a pilot valve having desirable operationalcharacteristics that is relatively simple and inexpensive tomanufacture.

Another .object is to provide a practicable and 5 inexpensive pressureregulating valve unit of pilot l type;

" Other more specic advantages and features of the invention will appearfrom the detailed description to follow which refers to the drawing. YPressure regulating valves are commonly used in hydraulic power systemsto maintain the pressure in the system between predetermined limits byconnecting the output of the pump to the systemwhen the system pressuredrops below a predetermined lower limit and disconnecting the output ofthe pump from the system when the system pressure rises above apredetermined upper limit. Such a system is disclosed in the TrautmanPatent 2,320,763, previously referred to. As shown in that patent, theoutput of the pump is connected through a `check valve to the hydraulicsystem and is connected through a normally closed hydraulically actuatedvalve to the fluid reservoir. so that, when the latter valve opens, thepump can discharge directly back to the reservoir at low pressure anddoes not Waste energy pumping fluid against a high head pressure. In thepatent, the hydraulically actuated valve is controlled by a pilot valvewhich is responsive to the pressure in the system. The pilot valveconstruction disclosed in the patent has been used extensively but hasthe disadvantage that although it opens suddenly, it closes gradually."

This causes a corresponding slow closing of the hydraulically actuatedvalve, which is less desirable than a faster action.

The present invention represents an improvement over the system of theTrautman patent in that the pilot valve inherently operates with a snapaction in both its opening and closing movements. Furthermore, this snapaction is obtained without the use of mechanical contrivances, by sodesigning the valve that when it starts to open the pressure forcetending to open it is immediately increased, and when it begins to closethe pressure force opposing lthe closing force is suddenly decreased. yThese variations in the pressure forces tending to open the pilot valveare obtained .by making the controlling pressure effective over alargerr piston area after the valve has started to open than before itopened, and decreasing the effective ypiston area to which the pressurefluid is applied when the valve begins to close.

In the drawing:

Fig. 1 is a plan view of a, pressure regulator valve incorporating apilot valve in accordance with the invention;

Fig. 2 is a vertical sectional view taken in the plane II-II of Fig. 1;l

Fig. 3 is avertical section taken in the plane III- III of Fig. 1; n

v Fig. 4 isa horizontal section takenlin the plane IV--IV of Fig. 2;

Fig. 5 is a horizontal section taken in the plane V--V of Fig. 3;

Fig. 6 is a schematic diagram showing the valve incorporated in ahydraulic system; and

Fig. '7 is a graph illustrating the operation of the valve.

The general construction and operation of a valve in accordance with theinvention will iirst be described with'reference to the schematicdiagram of Fig. 6 and the graph of Fig-7, after which an actualconstruction of the valve will be described with reference to Figs. 1 to5, inclusive.

Referring to Fig. 6, there is shown a system including a valve Ill, auid reservoir II, a pump I2, and an accumulator I3. The pump I2 isadapted to pump fluid from the reservoir II through a conduit III anddeliver it through a pump output conduit I5 to the valve I0. Anv exhaustconduit I6 extends from the valve I0 back to the reservoir II, and apressure conduit I1 extends yfrom the valve I0 to the system to besupplied with pressure iluid, the accumulator I3 being floated on thepressure conduit I1. The accumulator I3 isy not always employed, itsfunction being merely to equalize pressure variations in the pressureconduit l1 and to maintain the pressure in the conduit for a longerperiod of time when the pump I2 is not supplying the demand.

The function of the regulator valve IB is to maintain the pressure inthe pressure conduit I'I between predetermined limits by connecting thepump output Conduit I5 directly to the exhaust conduit I5 when thepressure in. the conduit I1 exceeds a desired Value and reconnecting thepump output i5 to the pressure conduit I'I when the pressure valve dropsbelow a predetermined value. It is to be assumed that the pump l2 runscontinuously during operation of the system.

As shown in Fig. 6 the regulator valve I Q. is in condition fordelivering thte fluid flow through the pump output conduit I5,4 to. the@95.5138 GQH: duit Il. Thus the conduit I5 is connected to a valvepassage I8, which in turn is connected by a check valve 22 with a Valvepassage 23 which d is connected with the` pressure conduit I1,

lpressure conduit Il, gradually building up the pressure therein.

Thel pressure. in the passage 23 is, applied through a branch passage23a to the lower end of a bore 24, containing aV piston element 2,5having a head 25a which is sealed with the bore 2,4 by aV seal 25h. Thepiston element` 251 has an D.

axial passage 25e therein, the lower edge 25d,` of which constitutes aseat for a pilot poppet 2 6, 'which is lightly urged against the seat:25d by. a hel-ical compression spring 2l..

rEhe upper end ot the bore 24 merges into the lower end of a bore 28 ofslightly larger diam-1 eter, and the upper end portion geof'the pistonelement 25V is sealed with respect to the bore 28 by a seal 25j. Thepiston element 25 is; urged downwardly by a helical loading spring 29,which is compressed between the upper end of the piston element and theupper; end oi?` the bore. 23; 'lbefupper endl Qi. the piston passage2.50: merges into the lower end of a bore 25g, in, the' piston element.and. a normally Stationarxstem Sliprojustable stopI which is. threadedintothe. valve body. A seal; 38h prevents fluid leakage past the stem tothe exterior oi the body. The:V stem 3;@ has. an axial passage Sucextending, therein from its lower end and communicated at its-,upper end'with the. bore 28., which bfneisin.0011stantcom..-v munication with theexhaust conduit IB through apassage 2 la. The lower endof the passage.3.80 constitutes a valve seat 30d adapted tobe closed; bylan auxiliarypoppet 26a formed on the upper end of a stem 2611. on the pilot poppet23. The lower eHdpf the. bore 25g. inthe piston element 25iscommunicatedby radial passages 25h with the. lowerend of the bore 27B,which` is coing.- municated by-a passage .32" with thelowel; end

.of the cylinder I9, which contains.- apiston/ 34,

pressure is applied through the passage 23a to the lower end of the bore2li and is effective against the lower end of the piston element 25 andthe pilot poppet 2B, to urge them upwardly against the force of theloading spring 29. When the pressure reaches a predetermined valuedetermined by the strength of the spring 29 and the position of thestationary stem 36, the auxiliary poppet 26a seats against the seat 30d,thereby breaking connection between the lower end of the cylinder I9 andthe exhaust passage 2|. Previous to this time, the piston 34 was exposedto exhaust pressure on both sides, and the main poppet 20 wasvmaintained closed by its spring ima. The closure of the auxiliary poppet26a on the seat 30d does not immediately change the pressure in thelower end of the cylinder IB, but it does prevent further upwardmovement of the pilot poppet 2E. The uid pressure acting against thepilot poppet 26 is no longer eiective to urge the piston element 25'upwardly, and the latter remains stationary until the pressure in thepase sage 23a rises to a higher Value such that the force created bythis pressure against the annular lower end of the piston element 2,5 issuiiicient to alone overcome the. spring 29,. Thereupon, the seat 25d iscracked away from the pilot puppet. 26, permitting pressure fluid toflow upwardly through the passage 25C, the passages 25h and the passage32, to the lower end of the cylinder IS, starting movement of the piston34A upwardly to open the main poppet 20. Following cracking of the scat25d; away` from the pilot poppet 26; the piston element 25 is movedrapidly upwardlyl to produce a wide opening between the pilot4 poppet`26: and the seatV 25d, because ofl the fact, that. the pressure fluidadmitted through the radial passages 2571.v to the lower endv of thebore 28 is eective; against; the annular face 2'52' of the large headportion 25e of the piston element, the diameter of which is larger thanthat of the lower head portion 25a of the piston element. Hence, as soonas the seat 25d is cracked'. awayY from the pilot poppet 26, anadditional upward force is. applied to theA piston element, this upwardforce being.- proportionalto the difference. between the areasof thebores- 24 andi 2d. respectively.

The sudden upward movement of the piston element 2,5 and consequentlarge opening between the pilot poppet 2S and its. seat 25d produces asuddenv rush off pressure uid thro-ugh passage 32 into the lower. end ofthe, cylinder I9, which suddenly opens .the main poppet Zogwide. There-lafter, the output of the pump I 2.. flows directly through thepassageI8l past the poppet l2l) and' through the exhaustpassagev EI andtheexhaust conduit I6; back to the reserVoinsothatthe load isremoved fromthe pump.

Assuming thatuid isbeing withdrawn by the system from thepressureconduit Ilz, the. pressure therein and in the lowerI end ofthe bore2`lwi1l gradually drop, following Opening of. the main; poppet 20, andthe piston element 2%iis gradually: returned to its lower position bythe loading spring 29. Eventually, this return movement carries the seat25d against the pilot puppet. 26, therebybreaking the connection betweenthe passage 23a and the passage 32, leading tothe lower end of thecylinder I9. However, pressure fluid is trapped in the passage 25e, bore25g, passages 2-5h, the passage 32, andthe lower endo cylinder I9, sothat the piston 3.4 still holds the main puppetl 2li open.Furtherdownward moi/.ef ment of thepiston element 25 following contactoi theseat Zidwith the. pilotpoppet 26 is tem.

porarily interrupted by virtue of the fact that the force of thepressure iiuid in the lower end ofthe bore 24 against the pilot poppet25 then becomes effective against the piston element 25. However, whenthe pressure in the pressure conduit II, passages 23, 23a, and the lowerend of bore 24 reaches a lower predetermined value, the spring `29 iseffective to move the piston element 25 and the pilot poppet 25downwardly, carrying the auxiliary poppet 26a clear of the seat 35d.Thereafter, the piston element 25 is moved suddenly, or snapped, intoits full lower position, because the opening of the auxiliary poppet 26aaway from the seat 30d releases the pressure fiuid that was trapped` inthe lower end of the bore 28 and which was effective against the pistonface 251'. The pressure in the passage 32 and the lower end of thecylinder I9 is therefore suddenly released, permitting the spring 25a toclose the main poppet 2i! and force the duid delivered by the pumpthrough output conduit I5 past the check v-alve 22 into the pressureconduit I 'I. The cycle of operations described will then be repeatedwhen the pressure in the pressure conduit I 1 again rises to a valuesuiiicient to crack the valve seat 25d away from lche pilot poppet '25.

The operation of the system may be readily followed by means of thegraph shown in Fig. 7, in which the curve 45 represents the pressure inthe pressure line Il, and the curve 4I represents l the pressure appliedto the main valve actuating piston 34.

The curve 40 shows the iiuctuations of pressure in the pressure line il'when fluid is being withdrawn from the pressure line at a rate less thanthe capacity of the pump I2. Beginning at point a on curve 40, the pumpis supplying fluid to the pressure conduit and the pressure thereinrises at a uniform rate to point 40h, at

which pressure the valve functions as described to open the main poppet2B and by-pass the pump discharge directly back to the reservoir.Thereafter the pressure in the pressure conduit I'I gradually drops tothe point 46c, at which value the main poppet 20 is again closed, andthe cycle repeats.

Referring now to curve 4I, it will be observed that up to point 4Ib thepressure applied to the lower end of the piston 34 is zero because thelower end of the cylinder I S is connected through the passages 32 and25h, bore 25g, the passage 30e and the passage 2Ia, directly to theexhaust passage 2I. At a point 4Ia, the auxiliary poppet 26a closes onthe seat 35d and disconnects the lower end of the cylinder I9 from theexhaust passage 2I. However, this does not change the pressure of thefluid trapped in the passages connected to the lower end of cylinder I9.

At point 4Ib the seat 25d is cracked away from the pilot poppet 26 andis thereafter moved rapidly into fully open position in the mannerpreviously described, causing the pressure acting against the lower endof the piston 34 to rise rapidly t0 point 4 Ic, at which pressure theforce of the pressure uid on the piston 34 is sufcient to begin to openthe main poppet 25. After it once begins to open, the main poppet 20opens rapidly, and the corresponding rapid movement of the piston 34permits relatively rapid ow of fluid through the passage 25o, thepassages 25h and the passage 32 into the lower end of the cylinder I9,causing a resulting pressure drop in the latter to the point 4Id on thecurve 4I. Thereafter the pressure in cylinder I9 rises rapidly to thepoint 4 I e on curve M and then declines gradually with the pressure 61n the pressure conduit II, as represented vby that portion of the curve40 between points 40h and 40e.

At a point 4If,v the return movement of the piston element 25 inresponse to reduction of the system pressure reseats the seat 25dagainst the pilot poppet 26, but this causes no change in the pressurebelow the piston 34. At point 4Ig on curve 4I, the system pressure hasbeen reduced sufficiently to again permit the piston element 25 toresume its downward movement, opening. the auxiliary poppet 26a off theseat 30d, whereupon the piston element 25 is snapped into its lowerposition, permitting the pressure under piston 34 to drop rapidly tozero, as indicated at point 41h on curve 4I, which results in theclosing of the main poppet 20.

It is particularly to be noted that if the bore 28 were of the samediameter as the bore 24, the opening of the auxiliary poppet 26a off theseat 35d would not change the pressure forces acting on the pistonelement 25 in opposition to the spring 25, and the piston would not besnapped into its lower position following crackingof the auxiliarypoppet Za away from its seat 35d. Instead, the auxiliary poppet 26awould be moved very gradually away from the seat 30d, resulting ingradual discharge of fluid from the lower end of the cylinder I9 andresultant sluggish closing action of the main poppet 20. Likewise, ifthe bores 24 and 28 were of the same diameter, the cracking of the seat25d away from the pilot poppet 25 would not change the pressure forcesacting on the piston element 25, and the latter would admit fluid onlyvery gradually to the lower end of the cylinder I9, resulting insluggish opening movement of the main poppet 20.

It is also to be noted that pressure uid in the bore 25g acts againstthe shoulder 25k at the lower end of that bore in direction to opposeopening movement of the piston element 25. However, the valve is soproportioned that the area of the shoulder 25k is substantially lessthan the` differential area between the bores 24 and 28 ref spectively,so that the force of the pressure fluid acting against the shoulder 251is greater than the force of the pressure iiuid acting against theshoulder 25k.

It is desirable that the main poppet 20 open and close with fairly rapidmovement in order to respond quickly to pressure changes in the systemand maintain the system pressure within the desired limits.

Figs. 1 to 5 inclusive disclose a practical valve constructionincorporating the features of the valve shown schematically in Fig. 6.Corresponding elements in Figs. 1 to 5 bear the same reference numeralsas Fig. 6.

Referring to Figs. 1 to 5 inclusive, it will be observed that the valvehousing comprises: a body 50 having an upper end closure cap 5I and alower end closure cap 52, both of which caps are secured to the body 5I]by screws 53 and 54. The upper end closure cap 5I functions to close theupper ends of bores and 28. Bore 55 functions as a guide for the mainpoppet 20 and contains a restoring spring 20a for the poppet 2D. Asealing ring 51 is provided to effect a fluid seal between the bore 55and the upper end cap 5I.

The bore 28 (Fig. 3) has been described with reference to Fig. 6. Asshown in Fig. 3 the upper end of this bore is sealed by a plug 51 havingaA sealing ring 58, the plug being retained against the sealing cap 5Iby the loading spring 29. The

7 blug 51 has a bore Gil which guides the stem 30, the latter beingsealed with respect to the bore 60 by the sealing ring 36o. Theadjusting screw 3l is threaded through the end cap 5l and may be lockedin a desired position of adjustment by a lock nut B2.

The screw 3.! provides for adjustment, within limits, oi the pressuresat which the valve operates, by varying the vertical position of theseat 30d in the lower end of stem 3H. Obviously if the stem 3E islowered, the auxiliary pcppet Ziia will Contact the seat Sidv at a lowersystem pressure.-

ToV facilitate manufacture, the bores 2li and 28 (Fig. 3) are formed ina tubular insert 6d separate. from the fbody 50. Thus the bore 281 inthe body .v is oi' uniform diameter throughout except at the lower endof the body where lt is counterbored. The tubular insert Sli is. fittedat its upper end into the bore 281 and sealed with respect thereto Joy aSealing ring 65. Intermediate its ends, the insert il is provided withan outwardly extending flange 6l which extends into the counterbore S8at the lower end oi' bore 281 and is held against the shoulder of thecounter borevr hyV a sealing ring Gil, which is pressed against theflange 6l by the lower end cap 52.

The' piston element 25 is also preferably made iritwo parts, one ofwhichV forms the lower head 25o and the-other of which forms the upperhead 2de.Y The head element 25a also preferably has a separate tubularseat element 'Eil which is inserted in the head- Za and sealed withrespect thereto by the same chevron-type seal 25h that eiiects sealbetweenv the piston head 25a and the bore 2li. The chevron-type. sealEtf, which efects aseal between the piston head 25e and the bore 28,also effects la; sealK between the separate head :portion 25e `and themain part of the piston 25..

Ityvvill begobviousfrom inspection o Fig. 3 that by; removing the lowerend cap 52, the tubular element ed* and. all of its associated parts canbe removed asa unit.

ReferringA to Fig. 2', the main portion of the cylinder la is formed inthe body E, and the lower end of thev cylinder is constitutedby thelower end cap 52, which is sealed with respect to the cylinder i9 by asealing ring l2. Likewise, the chamber for the check Valve poppet 22 is-iormed by a bore 13 inthe body 5e, which here isin direct communicationat its lower end with the 'passage 23, which is formed in the end cap52. The passage 23 terminates in a threaded port 14; which is adapted tobe connectedl to the pressure line. Il of Fig. 6. The :pump outputpassage Hlv is formed directly in the body 5%. and terminates iny athreaded port- T5 adapted to be connected to the pump output conduit l5of' Fig. 6.. The exhaust passage 2l is likewise formed directly in thebody 5U and terminates in a threaded port 'lrada'pted to lie connectedto the exhaust conduit It' of Fig. 6;

`It will be apparent that' when the lower end cap 52 is removed, thepiston. 34 with its integral stem 33a can be removed from the cylinderi9, and likewise the check valve poppet 22 and its spring 22h can lberemoved iro-mths bore i3, It is also obvious thatv when the upper endcap` El' is re' moved; the main ,poppet Eil and its spring alla, can beremovedI (Fig. 2) and the filler blockr 57 andthe spring Y29 (Fig. 3)can be removed.

Although considerableV variation in the relative diameters of the bores24 and 28 can be tolerated, I f'o'und that a desirable snap action onclosing and open-ing movements of the piston eleil merit 25 are obtainedin a valve having the fol-e lowing dimensions:

it will he understood that some special situations 'may make itdesirable to have faster valve action, in which case the bore 23 can hemade larger relative to the bore' 24, or the shoulder Zilk can be madesmaller. On the other hand, if Iless rapid action is desirable, thereshould be less diierence between the diameters of the bores and 23, orthe shoulder 25k should .be larger.

It will be noted that the .passage through the bore 25o around thepuppet stem 25h offers more resistance to fluid low than do the passages25h. This is desirable because the passage around'tlie stem 2dorestrictsk only the ow oi pressure lid from the passage 23a into thebore 25g, and does not restrict the ow of exhaust fluid. if the passages25h were restricted, they would not only offer resistance to ow ofpressure uid from the bore 25o' into the bore 28, but would also offerresistance to the exhaust or" uid from the bore 2e to the bore 25g.lSome restriction to ow'of pressure uid past the poppet stern 26h is desii-able to prevent too rapid opening or" the main valve ai). On theother hand, it is desralbleto permit relative free flow of exhaust fluidfrom the lower end oi the cylinder i9 through the passage 32 thepassages 25h, and the passa-ge 30e in order to permit fairly rapidclosing of the main valve 2i).

As herein disclosed, the pilot valve, main pop- `pet and check valve areall incorporated in a single valve body in order to make a compactassembly. However, it is not necessary that the main poppet 20 and itsactuating piston and cylinder and the check valve 22 he incorporated inthe same body with the pilot valve mechanism, and the prior TrautmanPatent 2,320,763 referred to herein above discloses these elements assep arate units. Where, in the claims, the :pilot valve alone isclaimed, the passages 21a, 32, and 23a of Fig. 5 are referred torespectively as a low pressure port, a control port, and a high pressure.port respectively, to simplify the claims.

Although for the `.purpose of explaining the invention, one particularconstruction thereof has been describedl in detail, numerous departuresfrom the exa-ct construction shown can be made While still utilizing theadvantages yielded by the invention, which is to be limited only to theex;- tent set. forth inthe appended claims.

I claim:

l1. In a hydraulically actuated device: means defining a steppedcylinder having a section of small diameter at one end and a section oflarge diameter at theother end; a stepped pistn with; ih said cylinderhaving a sinall head sealing iti the srhall section of said cylinder,and having e large headsealing in the large section of said cylinder; ahigh pressure port connected to the small end of said cylinder and a lowpressure port connected to the large end-of said cylinder; spring meansurging said piston toward the small ena of saidl cylinder; said pistonhaving a predetiif mined neutral position intermediate the ends of itsoperating stroke; valve means responsive to the position of said pistonfor connecting the inner end of Said large section of said cylinder portin response to movement of said piston from said neutral position towardthe large end of said cylinder, and for connecting the inner end of saidlarge section of said cylinder between said piston heads to said lowpressure port in response to movement of said piston from said neutralposition toward the small end of said cylinder; whereby said piston isurged away from said neutral position in either direction with increasedforce when once started away from said neutral position by a variationin the relative pressures in said low and high pressure ports.

2. A device Ias described in claim 1 in which said valve meanscomprises: a passage extending axially through said piston; a stationaryhollow stem extending into said piston passage from the large end ofsaid piston in sealing relation therewith, said stem having a passagefor communieating said piston passage with the large end of saidcylinder beyond said piston; a valve adapted to close said pistonpassage at the small end of the piston, an auxiliary valve connected tosaid valve and adapted to close said stem passage; said valve closingsaid piston passage and said auxiliary valve closing said stem passagewhen said piston is in said neutral position.

3. A device as described in claim 1, having a control port connected tothe inner end of said large section of said cylinder between said pistonheads for supplying pressure iiuid to, and venting pressure from acontrolled device in renspose to movement of said piston in oppositedirections from said neutral position.

4. In ya hydraulically actuated device: means dening a stepped cylinderhaving a section of small diameter at one end and a section of largediameter at the other end; a stepped piston within said cylinder havinga small head sealing in the small section of said cylinder and having alarge head sealing in the large section of said cylinder, said pistonhaving a passage extending axially therethrough; a stationary hollowstem extending into said piston passage from the large end of saidpiston, said stem having a stem passage communicating the piston passageat the end of the stem with the large end of said cylinder beyond saidpiston; a valve adapted to close said piston passage at the small end ofthe piston; an auxiliary valve connected to said valve and adapted toclose said stem passage; a high pressure port connected to the small endof said cylinder beyond said piston, a low pressure port connected tothe large end of said cylinder beyond said piston, and a control portconnected to the inner end of the large section of said cylinder betweensaid piston heads; spring means urging said piston toward the small endof said cylinder beyond said piston; said piston having a predeterminedneutral position intermediate the ends of its operating stroke in whichsaid valve closes said piston passage and said auxiliary valve closessaid stem passage; said valve being responsive to the position of saidpiston for connecting the inner end of said large section of saidcylinder between said piston heads to the outer end of the small sectionof the cylinder beyond said piston in response to movement of saidpiston from said neutral position toward the large end of said cylinder,and said auxiliary valve being responsive to the position of said pistonfor connecting the inner end of said large section of said cylinderbetween said piston heads to the large end of said cylinder beyond saidpiston inV response to movement of said piston from said neutralposition toward the small end of said cylinder; whereby said piston ismoved rapidly away from said neutral position in either direction whenonce started away from the neutral position by variation in the relativepressures in said low and high pressure ports, and said control port isquickly connected to either said low pressure port or said highpressureport, according to the direction of movement of the piston.

5. In a hydraulically actuated device: piston means having rst andsecond pressure areas, both responsive to pressure thereagainst forurging said piston means in one direction; spring means urging saidpiston means in the other direction; a high pressure port and meansconnecting it to said rst piston area; said piston means having apredetermined neutral position intermediate the ends of its operatingstroke; valve means responsive to the position of said piston means foradmitting pressure iluid from said pressure port to said second pistonarea in response to movement of said piston means from said neutralposition in said one direction; and valve means for venting pressure uidfrom said second piston area in response to movement of said pistonmeans from said neutral position in said other direction; whereby saidpiston means is moved rapidly away from said neutral position in eitherdirection when once started away from the neutral position by variationin the pressure in said pressure port.

6. A device as described in claim 5 having a control port communicatingwith said second piston area for supplying pressure fluid to, andventing pressure fluid from a controlled device in response to movementof said piston means in opposite directions from said neutral position.

FREDERICK W. SCHNECK.

REFERENCES CITED The following references are of record in the le ofthis patent:

UNITED STATES PATENTS Number Name Date 2,264,375 Hill Dec. 2, 19412,320,763 Trautman June 1, 1943 2,375,411 Grant May 8, 1945

